An output flow/pressure closed loop controller 110 is shown in a circuit diagram in FIG. 6 and is illustrated in detail in FIG. 7 using symbols.
The complete output flow of a pump 111 is herein passed through a variable throttle or orifice. For a fixed position of the throttle 2 the pressure drop at the throttle 2 is proportional to the output flow. Therefore the pressure drop can be considered as being indicative for the output flow. The pressure drop (or differential pressure) acts on the two end faces of a first control spool 4 of an output flow closed loop controller (or output flow control valve) 112.
The control spool 4 has a fixed operating position which is reached when a control opening 6 formed by a control edge a of the spool 4 and a wall b of the bore of the housing is about equal in size as an input flow nozzle 7, i.e. when the edge a of the spool 4 is just opening bore b. By dividing the pressure among the input flow nozzle 7 and the variable output flow opening 6 the pressure acting on the control spool can be equal to the pump pressure (with the opening 6 closed) at the most and at least equal to the reservoir pressure (with the opening 6 completely open).
If in this position of the spool 4 there is an equilibrium of forces, the output flow control operates in the stationary range. If the operating pressure determined by the load 3 is less than the pressure set by a spring 12 of a pressure closed loop controller (pressure control valve) 113 and a valve 15, then a control opening 13 of the pressure control valve 113 is closed and is disregarded in the present contemplation. Now, if, for example, the free cross section of the throttle 2 is made smaller, then, with the present output flow, the pressure drop at the variable throttle 2 is increased and the output flow control spool 4 is moved against the increasing force of a compression spring 5. Thus the control opening 6 increases, the hydraulic oil flows with less resistance to the reservoir, and the pressure at a control piston 9 of the pump 112 decreases.
A control piston 10 opposite the control piston 9 then moves a cam ring 11 of the pump 111 towards smaller displacement until the pressure drop at the variable throttle 2 again corresponds to the biasing of the spring 5. When the load pressure increases, the increasing leakage of the pump 111 is regulated until the second or pressure control spool 14 of the pressure closed loop controller 113 opens the control opening 13. The hydraulic oil now flows in parallel to the control opening 6 to the reservoir and the pressure at the control piston 9 decreases causing the pump 111 to decrease its output flow. The pressure at which the pressure closed loop controller 113 opens the control opening (i.e. the cut-off pressure) is determined by controlling the solenoid operated valve 15.
The use of this output flow/pressure closed loop controller enables the exactly repeatable electrical control of the pressure and the output flow. However, both of the closed loop controllers, i.e. the output flow closed loop controller 111 and the pressure closed loop controller 113, are specially designed. The output flow closed loop controller 111 is superimposed to the pressure closed loop controller 113. Both closed loop controllers 111 and 113 are provided with hydraulic oil via one common nozzle 7.
It is therefore disadvantageous in this known output flow/pressure closed loop controller that special components are required. This is particularly true for the output flow control valve 111 and the pressure control valve 113. A rigid arrangement of the system is provided where no standard components can be used. Further, the prior art output flow closed loop controller 110 is less dynamic than a standard pressure closed loop controller, this being particularly prominent in the control-up being slow.